In recent years, in order to cope with environmental issues with increased severity, a severe imposition of reduction in fuel consumption is increasing in each of the technical fields. Similarly, a more contribution to energy conservation is also strongly demanded on lubricating oils for automatic transmission.
As means for reducing fuel consumption of a lubricating oil for automatic transmission, an increase of a transmission torque capacity, namely suppression of an energy loss in a wet type clutch portion is basically important.
In general, the transmission torque capacity is attained by minimizing slippage of a wet type clutch during driving. It is meant that this is attained by a lubricating oil having a large coefficient of dynamic friction between the wet type clutches (for example, μ1800: coefficient of friction at the time of 1800 rpm). In consequence, it is basically required to develop such a lubricating oil for automatic transmission.
As a matter of course, in order to minimize a gear change shock, it is also indispensable to minimize (coefficient of static friction)/(coefficient of dynamic friction), for example, μ200/μ1800.
Furthermore, as other means for reducing fuel consumption of a lubricating oil for automatic transmission, there is a method of contriving to lower a viscosity of the lubricating oil to reduce agitating resistance, thereby contriving to enhance fuel efficiency.
However, when the viscosity of a lubricating oil is lowered, a more lowering of the viscosity is accompanied in a high-temperature region, so that capability of forming a lubricating oil film is greatly lowered. As a result, metal fatigue of a sliding member of an automatic transmission is caused, so that there is generated a concern that durability of the transmission cannot be retained, for example, impossible control of gear change, or the like.
Also, in the automatic transmission, there is a concern that a low-viscosity lubricating oil causes oil leakage in a hydraulic control part of the transmission, whereby the control of gear change becomes impossible.
In the light of the above, it is a difficult problem to make an enhancement of fuel efficiency by lowering the viscosity of a lubricating oil and an enhancement of fatigue resistance (durability) of an automatic transmission per se compatible with each other.
Furthermore, since an automatic transmission fluid is broad in a use temperature range, a lubricating oil with a high viscosity index is required. However, it is difficult to obtain a lubricating oil having satisfactory shear stability and capable of retaining a stable viscosity.
With respect to lubricating oils for transmission such as an automatic transmission fluid and the like, for example, there has hitherto been disclosed a lubricating oil composition for transmission containing a low-viscosity base oil (a kinematic viscosity at 100° C. is from 1.5 to 6 mm2/s) and a poly(meth)acrylate not containing a long-chain alkyl group in a side chain thereof, which is considered to be a lubricating oil for transmission which has a low viscosity and which, despite this, is excellent in fatigue resistance or the like (see Patent Document 1).
But, in such a composition, even when the viscosity index is not thoroughly high (not more than 170), a lowering of the viscosity of the lubricating oil at a high temperature is large, so that there is a concern that the fatigue resistance is lowered. Also, an increase of the viscosity of the lubricating oil at a low temperature is large, so that there is a concern that the composition is inferior in fuel efficiency. Furthermore, transmission torque capacity, ability for preventing a gear change shock and the like are not investigated, and there is room for more improvements.